Available pump NPSH

Estimate available suction head above vapor pressure from a large source vessel.

How this calculation works

Available net positive suction head measures the suction energy above the liquid's vapor-pressure basis in this large-vessel model. Enter absolute pressure at the vessel surface, vapor pressure at the operating temperature, surface elevation relative to the pump datum, suction-line head loss and liquid density. The pressure difference is converted to a liquid head, then elevation is added and losses are subtracted. Positive elevation represents a surface above the datum; a lower surface has a negative elevation. Surface velocity is assumed negligible, and all relevant suction losses must be supplied. Compare the result with the manufacturer's required NPSH and an application-specific margin. The calculation does not supply that margin or guarantee freedom from cavitation. Transients, entrained gas and suction recirculation are not modeled.

Inputs and units

Vessel surface absolute pressure (kPa): Use absolute pressure, not gauge pressure.
Liquid vapor pressure at temperature (kPa)
Surface elevation above pump datum (m)
Suction-line head loss (m)
Liquid density (kg/m³)

Method and formula

NPSHa = (Psurface,abs−Pvapor)/(ρg) + zsurface−hsuction loss, with pressures converted from kPa to Pa, g = 9.80665 m/s² and elevation relative to pump datum.

Worked example

Example inputs

Vessel surface absolute pressure: 101.325 kPa
Liquid vapor pressure at temperature: 2.339 kPa
Surface elevation above pump datum: 2 m
Suction-line head loss: 1 m
Liquid density: 998.2 kg/m³

Calculation steps

Pressure above vapor pressure = 101.325 − 2.339 = 98.986 kPa = 98986 Pa.
Surface pressure margin head = 98986 / (998.2 × 9.80665) ≈ 10.111964 m.
Available NPSH = 10.1119644418 + 2 − 1 ≈ 11.111964 m.

Example results

Available NPSH: 11.11196444 m
Surface pressure margin head: 10.11196444 m

Example notes

Compare with the pump manufacturer’s required NPSH and application-specific margin; this result does not certify freedom from cavitation.

Assumptions

Large vessel with negligible surface velocity; all suction losses included.
Vapor pressure and density match the liquid and operating temperature.

Limitations

No NPSHr database, transient, entrained-gas or suction-recirculation assessment.
A negative result diagnoses an invalid operating assumption, not available operating capability.
Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

Xylem/Goulds Water Technology Technical Manual: Printed page 11: open/closed source vessel NPSHa, suction head/lift and losses
DOE Fundamentals: Fluid Flow: Continuity and Bernoulli energy balance; centrifugal pumps, equations 3-19 through 3-25; series and parallel piping